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EXPLORING SEASONAL CLIMATE VARIABILITY IN EAST AFRICAN TREES: HIGH-RESOLUTION OXYGEN AND CARBON ISOTOPE RECORDS OF CELLULOSE OF VITEX PAYOS FROM CHYULU HILLS, KENYA
by
Patrick Devin Horan
A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
May 2011
Copyright 2011 Patrick Devin Horan

A nearly continuous, annually resolved record of cellulosic δ¹⁸O and δ¹³C is presented for the East African tree Vitex payos for the recent through the late 19th century. The isotope data of individual rings is cross-calibrated against atmospheric variables during the late 20th century to ascertain how and when during the year cellulose obtains its δ¹⁸O and δ¹³C value in response to environmental conditions at the tree site. During the late 20th century the δ¹⁸O and δ¹³C obtained from alpha cellulose from annual rings of V. payos correlates strongly with precipitation amount and with relative humidity, with depleted (enriched) δ¹⁸O and depleted (enriched) δ¹³C values associated with increased (reduced) precipitation and relative humidity. The strongest correlations are found between cellulose δ¹⁸O and the amount of precipitation and the relative humidity during December-February, although temperature contributes to the observed isotopic variance as well. Relative humidity in December-February appears to have the strongest correlation to annual cellulosic δ¹³C values. The results of this cross-correlation study are used to develop a preliminary seasonal growth model for V. payos that reflects variations in precipitation amount and relative humidity during the December-February growing season.; During the late 20th century the East Africa continent and the western Indian Ocean basin experienced anomalously wet conditions in summer (December-February) when sea surface temperatures (SSTs) were anomalously warm in the western Indian Ocean and zonal winds were anomalously strong in association with positive phases of the Indian Ocean Dipole (IOD) and/or El Nino Southern Oscillation (ENSO). Using the correlation between cellulosic isotopic values and hydroclimate variability over East Africa and Indian Ocean SSTs during the late 20th century a preliminary climate reconstruction is proposed for the pre-20th century from V. payos δ¹⁸O and δ¹³C. The isotopic record of V. payos indicates the late 19th century was anomalously wet over East Africa when compared to the late 20th century, consistent with lake level changes and ice core isotopic data. Climate model simulations indicate the wetter pre-20th century climate was a response to enhanced warming in the western Indian Ocean basin, generating a low pressure center and easterly surface winds that brought moisture to East Africa. The annually resolved isotopic data from V. payos suggest much of the observed and modeled hydroclimate variability at multidecadal to century scales is a response to SST anomalies in the western Indian Ocean.; The results of the present study should help guide future phenological studies of V. payos and other Vitex species in equatorial East Africa. Importantly, stable isotope data obtained from individual growth rings suggests that the annual production of cellulose in V. payos occurs during a short seasonal period from January to February, information that could not be inferred directly from tree ring width data alone. Further improvements in sampling tree rings at intra-annual resolution and in-situ monitoring of weather/climate variables, including isotopic variability of moisture would be necessary in order to fully utilize cellulosic δ¹⁸O as a climate proxy, with the potential for becoming a continent-wide hydrologic monitoring network, the first of its kind from Africa. The isotopic database for V. payos is among the few available terrestrial climate records provides estimates of how hydrologic budgets varied over East Africa during the past several centuries.

EXPLORING SEASONAL CLIMATE VARIABILITY IN EAST AFRICAN TREES: HIGH-RESOLUTION OXYGEN AND CARBON ISOTOPE RECORDS OF CELLULOSE OF VITEX PAYOS FROM CHYULU HILLS, KENYA
by
Patrick Devin Horan
A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
May 2011
Copyright 2011 Patrick Devin Horan